1. The Field of the Invention
The invention generally relates to systems and apparatus for overcoming discontinuities present in conductive pathways on optical transceivers.
2. The Relevant Technology
Fiber optic technology is increasingly employed as a method by which information can be reliably transmitted via a communications network. Networks employing fiber optic technology are known as optical communications networks, and are marked by high bandwidth and reliable, high-speed data transmission.
Optical communications networks employ optical transceivers in transmitting information via the network from a transmitting node to a receiving node. Generally, optical transceivers implement both data signal transmission and reception capabilities. A transmitter portion of a transceiver converts an incoming electrical data signal into an optical data signal, while a receiver portion of the transceiver converts an incoming optical data signal into an electrical data signal.
Depicted in FIG. 1 is an optical transceiver 100, which can transmit and receive modulated optical data. Transceiver 100 includes active and/or passive circuitry components connected to, or mounted on, a substrate 101 (e.g., a printer circuit board or “PCB”), the circuitry component being designed to implement transmitting and receiving functionality. For example, Transmitter Optical Subassembly (“TOSA”) 116 and Receiver Optical Subassembly (“ROSA”) 108 can be mounted on substrate 101 or connected to substrate 101 through a flex circuit. TOSA 116 further includes an optical signal source, such as, for example, a laser diode or Light Emitting Diode (“LED”), for generating modulated optical data. ROSA 108 includes a photodetector, such as, for example, a photodiode, for detecting modulated optical data.
Other active and/or passive circuitry components mounted on substrate 101 are designed to interoperate with TOSA 116 and ROSA 108 to facilitate transmitting and receiving modulated optical data. For example, optical transceiver 100 can also include eye safety component 122, controller integrated circuit (“IC”) 148, laser driver 118, post-amplifier 110, and memory components such as EEPROM 128, and so forth. Optical transceiver 100 can also interface with external conductive pathways through appropriate connections. For example, optical transceiver 100 can interface with an electrical transmit pathway (Tx+ and Tx− 120), an electrical receive pathway (Rx+ and Rx− 130), power connection 104, ground connection 106, etc.
Referring to transmitting modulated optical data, transceiver 100 receives a digital electronic input signal and converts the digital electronic input signal to an equivalent modulated optical signal. More specifically, laser driver 118 receives a digital electronic input signal received through TX+ and TX− 120 pathways (e.g., from pins connected to a computer system (not shown)). Based on the received digital electronic input signal, laser driver 118 drives the light source included in TOSA 116 to generate an equivalent modulated optical signal. Laser driver 118 includes an alternating current (“AC”) driver to provide AC current to the light source as well as a direct current (“DC”) driver to provide bias current to the light source
Referring to receiving modulated optical data, transceiver 100 receives a modulated optical signal converts the modulated optical signal to an equivalent digital electronic output signal. More specifically, the photodetector included in ROSA 108 receives a modulated optical signal and converts the modulated optical signal to an electrical signal. A pre-amplifier in ROSA 108 amplifies the converted electrical signal such that post amp 110 can detect and process the converted electrical signal. Post amp 100 amplifies and limits the converted electrical signal to generate a digital electronic output signal. The digital electronic output signal can have a uniform amplitude (or fixed swing) digital electronic output signal. The digital electronic output signal is presented at RX+ and RX− 120 conductive pathways (e.g., from pins connected to the computer system).
Post amp 110 can also provide a digital output signal known as Signal Detect (“SD”), or Loss of Signal (“LOS”), indicating the presence or absence of a suitably strong optical input. This SD output is provided via a SD output pin 114. Transceiver 100 can further include a controller IC 148 that, in conjunction with input/output (“I/O”) pins 150 (and associated circuitry), can provide certain functions to, for example, the post-amp 110 and laser driver 118.
In addition, some optical transceiver standards require additional transceiver functionality. For example, the GigaBit Interface Converter (“GBIC”) standard specifies the implementation of eye safety and general fault detection functionality. This functionality may be integrated into the laser driver IC 118 itself or into an optical eye safety IC 122. To enable this functionality, TX disable 124 and TX fault 126 pins are provided. The TX disable pin 124 allows the TOSA 116 to be shut off by a host, while the TX fault pin 126 communicates a fault condition in the laser, or associated laser driver IC 118, to the host device. In addition to this basic description, the GBIC standard includes a series of timing diagrams describing how these controls function and interact with each other to implement reset operations and other actions. The GBIC standard also defines the use of an electrically erasable programmable read-only memory (“EEPROM”) 128 to store standardized serial identification (“ID”) information that can be read via a serial interface consisting of clock 134 and data 132 lines.
Thus transmitting and receiving modulated optical data and performing related functionality requires that electrical signals be transferred along conductive pathways (also referred to herein as “conductor”) that interconnect the electrically conductive components of transceiver 100. Transferring an electrical signal through the several different conductive components on a conductive pathway can result in mismatched electrical impedances due to “discontinuities” between the different conductive components. Generally, a “discontinuity” is an element or region along a conductive pathway (e.g., a series of interconnected conductive components) that represents a change in shape of the conductive path. For example, a change in shape can occur when an electrical signal passes over one or more solder points on the conductive path between the laser driver 118 and the TOSA 116, over a junction from a PCB trace to a connector pin pad, from a bond wire to a lead frame or ball grid array substrate, and so forth.
At least one problem that a discontinuity presents is that each discontinuity in a conductive pathway causes at least a portion of a given electronic signal to reflect back onto the electrical signal, thereby disrupting the electrical signal. Reflections resulting from discontinuities can distort the rising and falling edges of electrical signals representing data bits such that the edges of the data bits no longer rise or fall as predicted. This distortion causes the data bits to move out of position, causing the data bits to arrive at the appropriate components out of order, such as arriving slightly earlier or slightly later than expected. This can cause the remaining data bits that arrive at the appropriate components at the appropriate time to be unintelligible. It may even be that a reflection is strong enough to make the top or bottom data bit bounce through a threshold at a receiver, causing data errors.
Reflections such as these can occur at varying degrees in a conductive pathway since there are a wide variety of possible discontinuities. In particular, the larger and more abrupt the discontinuity, the more power that is required to pass the electronic data signals through the given discontinuities from one point to the next. Hence, one conventional method for overcoming discontinuities is by adding power to the electrical signals. While this can work in low frequency systems that have relatively low energy requirements, systems that pass higher frequency data signals may not necessarily benefit from simply adding power to the signal. In particular, while simply adding power to a higher frequency electrical signal (e.g., an electrical signal representing data bits) can increase the amplitude of the electrical signal, the higher frequency data may be no more intelligible than before amplification.
Other conventional methods of overcoming reflections include adding impedance matching components along a conductive pathway. Generally, matching components are designed to reduce or eliminate reflections that occur by turning the reflections into heat in a resistor. Ideally, reducing the reflections to heat allows the electronic signal to pass through discontinuities without significantly muddying the data. Unfortunately, matching components require added power to operate effectively, and, moreover, reduce the amplitude of the electronic data signal in the process.
Furthermore, discontinuities can present a special problem to systems implementing high frequency data transmissions. For example, discontinuities tend to have greater significance when the length of the conductive pathway is much greater than the wavelength of the electrical signal representing the high speed data. By contrast, discontinuities tend to have less significance when the length of a given conductive pathway is much less than the electrical signal's wavelength. At 2.5 gigabits per second, for example, one electrical signal wavelength is approximately 6 cm. At 10 gigabits per second, the electrical signal wavelength is approximately 1-1.5 cm. This means that any conductive pathway longer than 1-1.5 mm (roughly 1 tenth of 1.5 cm) presents a particular problem for electrical signals representing data transmission in the 10 gigabit per second range.
Accordingly, an advantage can be realized with systems and methods that minimize the effect of discontinuities that can otherwise occur in data transmissions, particularly high-speed fiber optic data transmissions. In particular, compact systems that can minimize the distance of a high frequency conductive pathway, while consuming lower amounts of power would be an advantage in the present art.